1. Field of the Invention
The present invention relates to a printing apparatus for printing an image on a print medium by discharging ink droplets from respective ink orifices provided in a printhead based on image data, and a control method therefor, and particularly to a printing apparatus capable of obtaining a satisfactory image by correcting a shift of a dot forming position caused by a slant of a printhead, and a control method therefor.
2. Description of the Related Art
A general inkjet printing apparatus (to be referred to as a printing apparatus hereinafter) includes a printhead formed by arraying, in correspondence with each other, ink orifices and print elements each serving as an energy generation unit such as a heater or piezo element for discharging ink droplets. The printing apparatus discharges ink droplets to the print medium while moving a carriage mounted with the printhead in a predetermined direction (main scanning direction). Upon end of printing for one scan (printing scan), the printing apparatus conveys the print medium in a direction (sub-scanning direction: print element array direction) intersecting the main scanning direction. By repeating this operation, the printing apparatus completes image printing on the print medium. This printing is called serial printing.
Alternatively, there is provided a method of performing image printing while relatively moving the print medium and the printhead in the direction (sub-scanning direction) intersecting the array direction (main scanning direction) of the plurality of print elements mounted in the printhead.
It is not desirable for the printing apparatus to include a power supply necessary to simultaneously discharge ink droplets from all the ink orifices of each ink orifice array (print element array) of the printhead since the apparatus cost increases and noise is generated due to the flow of a large current. To solve this problem, conventionally, the plurality of print elements are time-divisionally driven.
Time-divisional driving is summarized as follows. A plurality of print elements forming each ink orifice array are divided into a plurality of groups each including a plurality of adjacent print elements, and the plurality of print elements included in each group are assigned to different blocks. The plurality of print elements of the respective blocks are sequentially driven at certain time intervals to drive all the print elements. This is called one driving cycle. In actual printing, printing is executed in a print region by repeating this cycle.
The printhead may be slanted and attached to the carriage of the printing apparatus due to a built-in error of the printhead and an attachment error caused when the printhead is attached to the printing apparatus. Consequently, the forming position of a print dot may shift in accordance with the slant. That is, a so-called shift by a slant may occur. This will be referred to as a printhead slant hereinafter.
Japanese Patent Laid-Open No. 2009-6676 proposes an arrangement of transferring print data, correcting a printhead slant by shifting print elements to be driven for each printing scan, and printing an image. Furthermore, Japanese Patent Laid-Open No. 9-104113 discloses an example in which a plurality of nozzles (print elements) are divided into a plurality of groups, and an image is formed while correcting a printhead slant by adjusting driving timings.
On the other hand, there is provided a method of arranging ink droplets on the print medium in line by adjusting ink discharge positions in correspondence with the above-described driving timings in order to improve the image quality of characters and thin lines.
FIGS. 44A to 44C are views showing the relationship between the driving timings of the printhead including 16 ink orifices and a dot arrangement on the print medium.
As shown in FIG. 44A, the ink orifices (orifices) are not vertically arranged in line in the array direction but arranged while shifting in a carriage moving direction. As is apparent from FIG. 44B, this shift corresponds to the above-described timings of time-divisional driving. Thus, discharge of ink droplets, and relative movements of the print medium and a printhead 11 make it possible to print a straight line, as indicated by dot positions on the print medium, which are represented by hatched circles in FIG. 44C.
The printhead 11 indicated by dotted lines in FIG. 44A represents a state in which the printhead 11 is slanted due to an attachment error to the printing apparatus, manufacturing variations, and the like. In printing in this state, it is impossible to print a straight line as described above, resulting in a slanted dot arrangement as indicated by dotted open circles in FIG. 44C.
In this state, the method proposed in Japanese Patent Laid-Open No. 2009-6676 adjusts, for example, the driving timings of print elements 200-0 to 200-7 included in an orifice group 200. However, even if such adjustment is performed, a printed dot group 2001 is only translated in a carriage moving direction while being slanted, and thus a shift of the landing position of an ink droplet occurs at the boundary between a dot which is translated and a dot which is not translated. As a result, no straight line is printed. Furthermore, when the printhead slant overlaps, on the print medium, a dot group printed by another printhead for discharging ink of a different color, a shift of dot coverage occurs due to the occurrence of a local shift in the dot arrangement, as described above, thereby causing band unevenness.
In addition, even if the printhead slant is corrected in accordance with the arrangement proposed in Japanese Patent Laid-Open No. 9-104113, the number of print elements which are driven at the same timing may change. The number of print elements which are driven at the same timing is defined as a “maximum concurrent drive number”. If this value is exceeded, discharge failure or image deterioration may occur due to a drive voltage drop of the printhead, and thus the value should be managed so as to not be exceeded. Furthermore, it is necessary to set the power supply capacity of the printing apparatus very large to make the maximum concurrent drive number changeable. This increases the apparatus cost.